FIG. 1 illustrates a prior an ignition system for a four-stroke four-cylinder engine requiring only one ignition coil. The ignition coil 14 comprises a primary 12 and secondary 16 winding. The primary winding 12 comprises series-connected first 12a and second 12b winding elements, the outer ends of which are connected to ground through two control transistors, 11 and 13, respectively. A battery 15 having a constant DC voltage potential is applied to an intermediate connecting point between the winding elements 12a and 12b.
Four spark plugs 26, 28, 30 and 32 having gaps formed therein are connected to the secondary winding 16 through diodes 18, 20, 22 and 24, respectively. Two spark plugs are connected to each end of the secondary winding, with their respective diodes arranged in reverse directions (oppositely poled) relative to one another.
Each spark plug gap is defined by two electrodes, an "earth" or "outer" electrode connected to ground and a "central" electrode connected to the secondary winding 16 through a diode.
The common end of both winding elements 12a and 12b is always at the DC voltage potential of the battery 15. With both transistors 11 and 13 off, the outer ends of the primary winding 12 are open-circuited so no current will flow through either winding element 12a and 12b.
When transistor 11 is turned on, the outer end of the first winding element 12a is effectively grounded, permitting current to flow through the first winding element 12a from the battery 15 to ground. The current flow produces a voltage potential in the first winding element 12a having a negative polarity at the grounded end with respect to the battery 15. As the current flow rises from zero to a steady-state value, a magnetic field is produced which induces a low-level voltage in the secondary winding 16 with the same polarity as that of the first winding element 12a.
When the transistor 11 is turned off, the magnetic field collapses, inducing a high-level voltage in the secondary winding 16 with a reverse polarity. The induced high-level voltage forward-biases two of the diodes, 20 and 22, causing arcing (i.e., discharge) between the electrodes of their respective spark plugs, 28 and 30. Conversely, similar operation of transistor 13 results in discharge of spark plugs 26 and 32. Therefore, selective operation of the two transistors, 11 and 13, provides a means of controlling the spark plug pairs to be discharged.
Note that in either mode of operation, the current discharge across the gaps of the two spark plugs is of opposite polarity, i.e., one of the two spark plugs being discharged has a negatively charged central electrode while the other has a positively charged central electrode.
Fuel is ignited near the end of the compression stroke of a cylinder by timed control of the appropriate transistor 11 and 13 to cause a current discharge across the electrodes of the spark plug 26-32 associated with the cylinder.
Since, as shown in FIG. 1, two spark plugs are discharged simultaneously, only one is for fuel ignition. The discharge of the other spark plug is during the exhaust stroke of its associated cylinder. Discharge of this spark plug serves no useful function and is therefore called the "waste-spark". It drains energy from the ignition system which would otherwise be used to discharge the compression-stroke spark plug.
The voltage drop across the waste-spark gap drains critical energy from discharge of the compression-stroke spark plug, requiring larger-sized ignition coils to compensate for the energy losses, adding to the overall cost of the ignition system.
FIG. 2 illustrates a prior an ignition system which eliminates discharge of the waste-spark. A single ignition coil 34 is provided for every two spark plugs, discharging only one of the spark plugs at any one time. The ignition system functions in much the same way as that described above for FIG. 1, except that one end of the secondary winding is grounded. As with FIG. 1, diodes 38 and 40 prevent the discharge of both spark plugs simultaneously. Only the compression-stroke spark plug is fired, thereby eliminating discharge of the waste-spark.
As with the ignition system of FIG. 1, the current discharge across the gaps of the spark plugs 42 and 44 are of opposite polarity in that discharge of one-half of the spark plugs within the engine is achieved by positively charging the spark plug central electrode and the other half by negatively charging the spark plug central electrode.
Optimally, the polarity of a spark plug central electrode during discharge should be negative. FIG. 3 illustrates a graph comparing erosion of positively and negatively charged central electrodes. As shown, erosion of spark plug electrodes is dramatically reduced (approx. 40% reduction) by discharging the spark plug with a negatively charged central electrode as compared to a positively charged one, increasing the useful life of the spark plug.
Additionally, a negatively charged central electrode breaks down the spark plug gap (i.e., achieves arcing) at a much lower voltage, thereby permitting reduced coil sizes and power requirements for a given application.
As can be seen, these desirable characteristics exist only with respect to one-half of the spark plugs in the systems of FIGS. 1 and 2.